Macrocycles are an important therapeutic class of antibiotics. These compounds are frequently produced as a family of closely related biogenetic congeners. The Tiacumicins are a series of 18-membered macrocyclic antibiotics in which the macrocyclic ring is glycosidically attached to one or two sugars. A seven-carbon sugar is esterfied at various positions with small fatty acids. The other sugar, when present, is esterified with an isomer of the fully substituted benzoic acid, everninic acid. (Journal of Liquid Chromatography, 1988, 11: 191-201).
Tiacumicins are a family of related compounds that contain the 18-membered ring shown in Formula I below.

At present, several distinct Tiacumicins have been identified and six of these (Tiacumicin A-F) are defined by their particular pattern of substituents R1, R2, and R3 (U.S. Pat. No. 4,918,174; J. Antibiotics, 1987, 40: 575-588), as shown in Table 1.
TABLE 1Substituents Present In Tiacumcins A-FR1R2R3AHH BOH COH DOH EOH FOH
Tiacumicins A-F have been characterized spectroscopically and by other physical methods. The chemical structures of Tiacumicins are based on spectroscopy: UV-vis, IR and 1H and 13C NMR, see for example J. Antibiotics, 1987, 40: 575-588. Inspection of Table 1 reveals that certain members of the family are structurally related isomers and/or differ by the presence or absence of certain moieties. Others differ in the nature of their ester groups.
Tiacumicins are produced by bacteria, including Dactylosporangium aurantiacum subspecies hamdenensis, which may be obtained from the ARS Patent Collection of the Northern Regional Research Center, United States Department of Agriculture, 1815 North University Street, Peoria, Ill. 61604, accession number NRRL 18085. The characteristics of strain AB 718C-41 are given in J. Antibiotics, 1987, 40: 567-574 and U.S. Pat. No. 4,918,174.
C. difficile-associated diarrhea (CDAD) is a disease characterized by severe and painful diarrhea. C. difficile is responsible for approximately 20% of the cases of antibiotic-associated diarrhea (AAD) and the majority of the cases of antibiotic-associated colitis (AAC). These diseases are typically caused by toxin producing strains of C. difficile, S. aureus including methicillin-resistant S. aureus (MRSA) and Clostridium perfringens (C. perfringens). AAD represents a major economic burden to the healthcare system that is conservatively estimated at $3-6 billion per year in excess hospital costs in the U.S. alone.
Vancomycin-resistant enterococci, for which intestinal colonization provides a constant reservoir for infection, has also emerged as a major nosocomial pathogen associated with increased health care cost and mortality. VRE can appear as coinfection in patients infected with C. difficile, or more commonly cause infection in certain high risk patients such as haematology and oncology patients, patients in intensive care units and patients receiving solid organ transplants.
Methicillin-resistant Staphylococci, such as MRSA, are increasing in prevalence in both the hospital and community settings. Staphylococci are found on the skin and within the digestive and respiratory tracts but can infect open wounds and burns and can progress to serious systemic infection. The emergence of multi-drug resistant Staphylococci, especially, in the hospital where antibiotic use is frequent and selective pressure for drug-resistant organisms is high, has proven a challenge for treating these patients. The presence of MRSA on the skin of patients and health care workers promotes transmission of the multi-drug resistant organisms.
Similar diseases, including but not limited to clostridial enterocolitis, neonatal diarrhea, antibiotic-associated enterocolitis, sporadic enterocolitis, and nosocomial enterocolitis are also significant problems in some animal species.
AAD is a significant problem in hospitals and long-term care facilities and in the community. C. difficile is the leading cause of AAD in the hospital setting, accounting for approximately 20% of cases of AAD and the majority of cases of antibiotic-associated colitis (AAC). The rising incidence of Clostridium difficile-associated diarrhea (CDAD) has been attributed to the frequent prescription of broad-spectrum antibiotics to hospitalized patients.
The most serious form of the disease is pseudomembranous colitis (PMC), which is manifested histologically by colitis with mucosal plaques, and clinically by severe diarrhea, abdominal cramps, and systemic toxicity. The overall mortality rate from CDAD is low, but is much greater in patients who develop severe colitis or systemic toxicity. A recent study has shown that even when death is not directly attributable to C. difficile, the rate of mortality in CDAD patients as compared to case-matched controls is much greater.
Diarrhea and colitis are caused by the elaboration of one or more C. difficile toxins. The organism proliferates in the colon in patients who have been given broad-spectrum antibiotics or, less commonly, cancer chemotherapy. CDAD is diagnosed in approximately 20% of hospitalized patients who develop diarrhea after treatment with such agents.
There are currently two dominant therapies for CDAD: vancomycin and metronidazole. Vancomycin is not recommended for first-line treatment of CDAD mainly because it is the only antibiotic active against some serious life-threatening multi-drug resistant bacteria. Therefore, in an effort to minimize the emergence of vancomycin-resistant Enterococcus (VRE) or vancomycin-resistant S. aureus (VRSA), the medical community discourages the use of this drug except when absolutely necessary.
Metronidazole is recommended as initial therapy out of concern for the promotion and selection of vancomycin resistant gut flora, especially enterococci. Despite reports that the frequency of C. difficile resistance may be >6% in some countries, metronidazole remains nearly as effective as vancomycin, is considerably less expensive, and can be used either orally or intravenously. Metronidazole is associated with significant adverse effects including nausea, neuropathy, leukopenia, seizures, and a toxic reaction to alcohol. Furthermore, it is not safe for use in children or pregnant women. Clinical recurrence occurs in up to 20% of cases after treatment with either vancomycin or metronidazole. Therapy with metronidazole has been reported to be an important risk factor for VRE colonization and infection. The current treatment regime against Gastrointestinal infections, e.g., Clostridium difficile-associated diarrhea (CDAD) is rather cumbersome, requiring up to 500 mg four-times daily for 10 to 14 days. Thus, there is a need for better treatment for cases of CDAD as well as for cases of other Antibiotic-associated diarrhea (AAD) and Antibiotic-associated colitis (AAC).
Tiacumicins, specifically Tiacumicin B, show activity against a variety of bacterial pathogens and in particular against C. difficile, a Gram-positive bacterium (Antimicrob. Agents Chemother. 1991, 1108-1111). C. difficile is an anaerobic spore-forming bacterium that causes an infection of the bowel. Diarrhea is the most common symptom but abdominal pain and fever may also occur. C. difficile is a major causative agent of colitis (inflammation of the colon) and diarrhea that may occur following antibiotic intake. This bacterium is primarily acquired in hospitals and chronic care facilities. Because Tiacumicin B shows promising activity against C. difficile, it is expected to be useful in the treatment of bacterial infections, especially those of the gastrointestinal tract, in mammals. Examples of such treatments include but are not limited to treatment of colitis and treatment of irritable bowel syndrome. Tiacumicins may also find use for the treatment of gastrointestinal cancers.
Tiacumicin antibiotics are described in U.S. Pat. No. 4,918,174 (issued Apr. 17, 1990), J. Antibiotics 1987, 40: 575-588, J. Antibiotics 1987, 40: 567-574, J. Liquid Chromatography 1988, 11: 191-201, Antimicrobial Agents and Chemotherapy 1991, 35: 1108-1111, U.S. Pat. No. 5,583,115 (issued Dec. 10, 1996), and U.S. Pat. No. 5,767,096 (issued Jun. 16, 1998), which are all incorporated herein by reference. Related compounds are the Lipiarmycin antibiotics (c.f., J. Chem. Soc. Perkin Trans. I, 1987, 1353-1359 and J. Antibiotics 1988, 41: 308-315) and the Clostomicin antibiotics (J. Antibiotics 1986, 39: 1407-1412), which are all incorporated herein by reference.